1. Field of the Invention
This invention pertains to isomerization processes and, more particularly, to processes whereby xcex3,xcex4-epoxyalkenes and xcex3,xcex4-epoxycycloalkenes are isomerized to obtain the corresponding 2,5-dihydrofuran compounds in the presence of a catalyst system comprising an organotin compound and an alkali metal iodide in the presence or absence of a solvent at a temperature of 50 to 200xc2x0 C.
2. Description of the Related Art
Dihydrofurans are reactive heterocyclic species which are useful in a variety of applications, e.g., as intermediates in the production of useful polymers and chemicals. However, the use of dihydrofurans for such purposes has heretofore been restricted due to the non-availability of cost-effective preparative procedures.
U.S. Pat. Nos. 3,932,468 and 3,996,248 disclose the production of 2,5-dihydrofurans by the rearrangement of substituted or unsubstituted epoxyalkenes with a homogeneous catalyst system comprising hydrogen iodide or hydrogen bromide and a transition metal Lewis acid in an organic solvent. This process suffers disadvantages of the use of corrosive hydrogen halides, high level of oligomer formation, and complications in product isolation. We have found that the process of U.S. Pat. Nos. 3,932,468 and 3,996,248 also results in the unwanted production of up to 15% xcex1,xcex2-unsaturated aldehydes or ketones.
U.S. Pat. No. 5,034,545 describes a method for the isomerization of epoxyalkenes to 2,5-dihydrofurans, in the liquid phase, in the presence of a catalyst system containing an alkali or alkaline earth metal halide or an onium halide, a Lewis acid, and an organic solubilizer. The best reported results were attained by using the combination of a zinc halide and an alkali metal halide. We have found that such combinations of catalyst system described in U.S. Pat. No. 5,034,545 gave very poor catalyst lifetime. The catalyst activity and the reaction of the epoxide decrease considerably after a short operation time and result in a high level of oligomer formation. Therefore, this method is uneconomical.
Furthermore, U.S. Pat. No. 5,315,019 discloses the isomerization of epoxybutenes in the liquid phase, wherein organotin and a tetraalkylammonium or phosphonium iodide are used as the catalyst system. European Patent No. 0 691 334 A1 discloses a method for the rearrangement of epoxybutenes to 2,5-dihydrofurans, in the liquid phase, in the presence of a Lewis acid and a phosphazenium halide or a phosphazanium halide in an organic solvent. Catalysts used in these methods are expensive. Some of them are not readily commercially available. Moreover, relatively large quantities of catalysts must be used in order to obtain satisfactory yields and selectivities.
The invention under consideration was thus based on the task of finding a catalyst system for the isomerization of epoxybutenes to 2,5-dihydrofurans, which is free of the aforementioned disadvantages.
We have discovered a catalytic process with a novel catalyst system for the isomerization of xcex3,xcex4-epoxyalkenes to produce dihydrofurans. The process provides high levels of epoxyalkene conversion with high selectivity to the desired dihydrofuran product. Long catalyst lifetimes are realized and the product may be recovered by relatively simple means since the catalyst and reaction mixture are readily separated by such simple techniques as distillation, decantation, filtration, gas stripping methods, gas/liquid flow separation, and the like. Catalysts involved in the process of this invention are readily obtainable by simple synthetic preparations or are commercially available.
In accordance with the present invention, there is provided a process for the isomerization of xcex3,xcex4-epoxyalkenes to the corresponding 2,5-dihydrofuran compounds, which process comprises contacting a xcex3,xcex4-epoxyalkene or xcex3,xcex4-epoxycycloalkene with a catalytic amount of an organotin compound in combination with an alkali metal iodide to catalyze the isomerization process of our invention under isomerization conditions of temperature and pressure.
The xcex3,xcex4-epoxyalkene and xcex3,xcex4-epoxycycloalkene reactants suitable for use in the process of our invention may contain from 4 to about 20 carbon atoms, preferably from 4 to about 8 carbon atoms. Examples of the epoxyalkene and epoxycycloalkene reactants include compounds having the structural formula (I): 
wherein each R1 is independently selected from hydrogen, alkyl of up to about 8 carbon atoms, a carbocyclic or heterocyclic aryl group of about 5 to 10 carbon atoms or halogen, or any two R1 substituents collectively may represent an alkylene group forming a ring, e.g., alkylene, containing in the main chain up to about 8 carbon atoms.
The preferred epoxyalkene reactants comprise compounds of formula (I) wherein only two of the R1 substituents individually may represent lower alkyl, e.g., alkyl of up to about 8 carbon atoms, or collectively represent straight or branched chain alkylene of up to about 8 carbon atoms. Exemplary compounds contemplated for use in the practice of the present invention include 3,4-epoxy-3-methyl-1-butene, 2,3-dimethyl-3,4-epoxy-1-butene, 3,4-epoxycyclooctene, 3,4-epoxy-1-butene, 2,5-dimethyl-2,4-hexadiene monoepoxide, and the like. The epoxyalkene reactant of primary interest is 3,4-epoxy-1-butene.
The 2,5-dihydrofuran compounds obtained in accordance with our novel process have the structural formula (II): 
wherein the R1 substituents are defined above. Of the compounds which may be obtained in accordance with our invention, the compound of primary interest is 2,5-dihydrofuran.
The preferred alkali metal iodides for use in the present invention include lithium iodide, sodium iodide, and potassium iodide. Lithium iodide and potassium iodide are particularly preferred.
The alkali metal iodide is used in combination with an organotin compound to catalyze the isomerization process of our invention. The organotin compound may be selected from organotin (IV) compounds such as hydrocarbyltin iodides, dihydrocarbyltin iodides, trihydrocarbyltin iodides, and tetrahydrocarbyltin compounds. Examples of such organometallic compounds include compounds having the formula: 
wherein each R2 independently is selected from alkyl or substituted alkyl moieties having up to about 20 carbon atoms, cycloalkyl or substituted cycloalkyl having about 5 to 20 carbon atoms, carbocyclic aryl or substituted carbocyclic aryl having about 6 to 20 carbon atoms, or heteroaryl or substituted heteroaryl moieties having about 4 up to 20 carbon atoms; and n is 1, 2, 3, or 4.
Specific examples of the organometallic compounds include dibutyltin diiodide, tributyltin iodide, trioctyltin iodide, triphenyltin iodide, trimethyltin iodide, butyltin triiodide, tetrabutyltin, tetraoctyltin, triphenyltin iodide, tribenzyltin iodide, dimethyltin diiodide, diphenyltin diiodide, tricyclohexyltin iodide, and dicyclohexyltin diiodide.
The preferred organometallic compounds comprise tin (IV) iodides having the above general formula and a total of about 2 to 36 carbon atoms wherein each R2 substituent independently is selected from alkyl of up to about 12 carbon atoms, benzyl, phenyl, or phenyl substituted with up to 3 substituents selected from lower alkyl, lower alkoxy, or halogen; and n is 2 or 3.
Some of the tetra-alkyl or -aryl substituted tin compounds may react with the alkali metal iodide co-catalyst under the conditions of isomerization to form in situ organotin iodide compounds. Such tetrahydrocarbyltin compounds include tetraphenyltin.
The amount of the organotin component of the novel catalyst compositions of this invention can vary substantially depending on the mode in which the isomerization process is operated, the particular organotin compound and alkali metal iodide present, etc.
The catalyst system is preferably employed in our process as an intimate mixture of one or more of the organotin compounds and one or more of the alkali metal iodides described hereinabove. The organotin compound:alkali metal iodide weight ratio of the catalyst system can vary substantially, e.g., from 200:1 to 1:100, depending on the particular catalyst components selected. The preferred organotin compound:alkali metal iodide weight ratio is about 50:1 to 1:50. Particularly preferred catalyst systems comprise a mixture of one or more of the organotin compounds described hereinabove and lithium iodide, potassium iodide, or sodium iodide.
The organotin compound and alkali metal iodide catalyst system may be used with an inert organic solvent, if desired, to alter the reaction conditions and/or reactor configuration. The optional, inert organic solvent may also be used to assist the catalytic process.
Thus, another embodiment of our invention comprises the isomerization of an epoxyalkene to the corresponding 2,5-dihydrofuran in the presence of a catalyst solution. This embodiment may be carried out in the presence of one or more of the above-described organotin compounds and alkali metal iodides. Accordingly, the catalyst solution preferably comprises a catalytic amount of (i) one or more of the above-described organotin compounds and (ii) one or more of the above-described alkali metal iodides in (iii) an inert organic solvent, i.e., a solvent that does not react with the xcex3,xcex4-epoxyalkene or xcex3,xcex4-epoxycycloalkene reactants or the 2,5-dihydrofuran products. Examples of the solvents which may be used include aliphatic and aromatic hydrocarbons such as heptane, toluene, specific or mixed xylenes, pseudocumene, and mesitylene; halogenated hydrocarbons such as chlorobenzene, 1,2-dichlorobenzene, and 1,1,2,2-tetrachloroethane; ketones such as cyclohexanone, 5-methyl-2-hexanone, and 2-heptanone; ethers such as 2,5-dihydrofuran, tetrahydrofuran, and bis(2-methoxyethyl)ether; esters such as isobutyl acetate; and tertiary amides such as N-methyl-2-pyrrolidinone, N-cyclohexyl-2-pyrrolidinone, N-ethyl-2-pyrrolidinone, and N,N-dimethylacetamide. Normally, for ease of separation, the solvent or mixture of solvents employed have boiling points at least 20xc2x0 C. above the boiling point of the 2,5-dihydrofuran product and the unsaturated aldehyde or ketone by-products.
The concentrations of the organotin compound and the alkali metal iodide in the inert, organic solvent can be varied substantially depending, for example, on the particular catalytically-effective components present, the design of the reactor system, etc. Typically, the concentration of the organotin compound will be about 1 to 70 weight percent and the concentration of the alkali metal iodide will be about 1 to 70 weight percent, both concentrations being based on the total weight of the catalyst solution. Normally, the mole ratio of alkali metal iodide to organotin compound is in the range of 50:1 to 1:50.
The preferred catalyst solution comprises:
(i) about 1 to 50 weight percent of an organotin compound containing a total of about 2 to 36 carbon atoms and having the formula: 
xe2x80x83wherein each R2 substituent independently is selected from alkyl of up to about 12 carbon atoms, benzyl, phenyl or phenyl substituted with up to 3 substituents selected from lower alkyl, lower alkoxy, and halogen, and n is 1, 2, 3 or 4;
(ii) about 1 to 50 weight percent of an alkali metal iodide; and
(iii) an inert organic solvent selected from tertiary amides such as N-methyl-2-pyrrolidinone, N-cyclohexyl-2-pyrrolidinone, N-ethyl-2-pyrrolidinone, and N,N-dimethylacetamide, with N-alkyl-2-pyrrolidones being particularly preferred.
Some organotin compounds, some alkali metal iodides, and the reaction mixture can be mutually soluble. The reaction mixture can include the epoxyalkene itself and mixtures of epoxyalkene, 2,5-dihydrofuran, epoxyalkene oligomers, and polymers which are formed as a consequence of the reaction. As a result, the addition of an inert organic solvent may not be necessary. This is particularly true when trioctyltin iodide and lithium iodide are used as the catalyst system. Moreover, if desired, a mixture of epoxyalkene with the reaction product 2,5-dihydrofuran and/or the epoxyalkene oligomers and polymers formed in the course of the reaction can be used in combination with, or in lieu of, the inert organic solvent in the reaction from the beginning.
The isomerization process may be carried out using the catalyst solutions described hereinabove by contacting a xcex3,xcex4-epoxyalkene or xcex3,xcex4-epoxycycloalkene at a temperature of about 50 to 200xc2x0 C., preferably about 80 to 160xc2x0 C., depending on the solvent or mixture of solvents employed. The process may be carried out at atmospheric or super-atmospheric pressures, e.g., up to about 22 bar (absolute).
The process employing the catalyst solution may be carried out in a batch, semi-continuous, or continuous mode of operation. For example, batch operation may comprise refluxing a mixture of the xcex3,xcex4-epoxyalkene and catalysts, e.g., tributyltin iodide and lithium or potassium iodide, in a solvent such as N-methylpyrrolidone for a time sufficient to convert essentially all of the epoxide to the 2,5-dihydrofuran. The products are then separated by distillation from the mixture. The undistilled catalyst solution may be reused in a subsequent reaction.
The catalyst solution preferably is employed in a continuous mode of operation wherein a xcex3,xcex4-epoxyalkene or xcex3,xcex4-epoxycycloalkene is added to a recirculated catalyst solution which is then introduced into a continuous reactor. After isomerization, the reaction stream is fed to a distillation system for removal of product or products and recycle of the catalyst solution. Examples of continuous reactor designs in which the process can be performed are continuous stirred tank reactors and plug-flow reactors.
Our novel isomerization process and the catalyst systems, compositions, and solutions useful in practicing the invention are further illustrated by the following examples. The conversions and selectivities reported in the examples were determined by gas chromatographic analyses performed on a Hewlett-Packard 5890 series II gas chromatography.